Meshblock Conflation¶
Overview¶
The NGD network needs to be linkable to the new CRN network. This includes individual arcs and BB (meshblock) units.
The CRN and NGD network differences are not limited to the quantity of arcs, but also the shape and alignment of those
arcs. This is why the networks cannot be simply linked arc-to-arc, but require more detailed conflation to reduce
alignment differences. A threshold of 80% is currently used to determine acceptable network alignment and classify
individual meshblock polygons (BBs) as conflated or unconflated.
Definition
conflated: An NGD BB which has >= 80% of its area contained within a single CRN BB.
Threshold Strictness
It will not always be possible to achieve the conflation threshold without introducing needless complexity to the
CRN. Therefore, the threshold should be treated as a soft restriction and lower values are acceptable so long as
the NGD BB can be guaranteed to link to a single CRN BB based on majority area occupation, as defined above for
conflation.
The required actions to resolve unconflated BBs should follow those outlined in the various validations in
Meshblock Creation.
Resources¶
- CLI Tool:
src/conflation/conflate_meshblock.py- Output (see data/crn.gpkg):
Basic metrics output to console.
Updated source layer:
crn_<source>New (CRN) BB layer:
<source>_meshblockCurrent (NGD) BB layer:
<source>_meshblock_ngd
- Editing Environment:
data/editing_meshblock_conflation.qgz
Editing Process¶
Figure: Editing process overview.¶
Cardinalities¶
Definition
Cardinalities: In a database context, cardinalities refer to the numerical relationships between records of two or more datasets.
Figure: Cardinalities overview. Direction: CRN (blue) - to - NGD (pink).¶
Cardinalities Explained:
one-to-one:- Explanation:
Networks align.
- Action:
None.
many-to-one:- Explanation:
CRN is more detailed.
- Action:
None.
one-to-many:- Explanation:
NGD is more detailed.
- Action:
Create new BOs and / or copy missing NGD roads.
many-to-many:- Explanation:
Networks conflict.
- Action:
Create new BOs.
QGIS Project¶
Explanation of Layers¶
Figure: QGIS table of contents.¶
Layers:
crn:Primary layer representing NRN roads, NGD BOs, and added NGD roads.
ngd_road:NGD roads for reference and identifying roads missing from the CRN which are required for conflation.
meshblock_ngd:Current NGD BB layer, classified by conflation validity status and labelled with a conflation percentage.
meshblock:New CRN BB layer generated from
crn, classified by conflation validity status and labelled with a conflation percentage.Esri Satellite:Reference WMS for recent imagery context.
Example Output¶
Conflation Scenarios¶
Converting NGD roads to BOs
If an NGD road needs to be converted to a BO, copy and paste the NGD feature(s) into the CRN data and set
bo_new=1. The script will then automatically set the correct segment_type for these features.
Adding new BOs (completely new arcs)
If a network difference prevents an CRN BB from being properly conflated, you may need to add a new arc to the CRN
data. After creating the arc, either set bo_new=1. The script will then automatically set to remaining
attributes.
Scenario: Missing NGD Roads¶
Scenario: Missing False NGD Road (1)¶
Scenario: Missing False NGD Road (2)¶
Scenario: Misaligned Networks¶
Note
Misaligned networks are difficult to resolve since there is no clear nor obvious solution. If the CRN and NGD BBs are not too different in shape, try modifying the BO shape slightly or rerunning the script with a slightly lower threshold value. Larger differences may require adding NGD roads as BOs and / or adding completely new BOs, thereby segmenting the data until the conflation threshold is satisfied.
Scenario: Misaligned Networks - Additional Examples¶
Scenario: Structural Differences¶
Note
A structural difference is where the CRN and NGD have different network representations (not the same as offsets / alignment issues).
Scenario: Isolated Blocks¶
Scenario: Correction of NGD Road¶
Note
Inaccurate / low resolution roads can be corrected based on the imagery layer, if desired, so long as the conflation threshold remains satisfied. However, this is not always possible (as shown below) and is not the objective of this task.
Progress¶
Note
This section is temporary and will be removed once this task is completed.
Figure: Meshblock conflation workload (error counts).¶
Figure: Meshblock conflation progress map as of November 18, 2022.¶